As the amounts of data involved rise ever further, the frequency separations between individual transmission frequency bands become ever narrower. In consequence, ever more accurate analog anti-aliasing filters are required. For data transmission, the data from a data source is converted by means of a digital/analog converter to an analog transmission signal, and is transmitted to a receiver via a data transmission channel. In the receiver, the received data is converted to digital data by means of a digital/analog converter, for further data processing. The analog anti-aliasing filters prevent the analog/digital converter and digital/analog converter from injecting undesirable signal interference into the transmission signal. The filters are generally integrated on a semiconductor chip. The analog anti-aliasing filters contain RC elements which comprise resistors and capacitors. When the filters are integrated on a semiconductor chip, process discrepancies occur during the production process, so that the resistance and capacitance values of the RC elements contained in the filter may not be the same as the desired nominal value. The time constant (τ) of an RC element is the product of the resistance R of the resistor and the capacitance C of the capacitor within the RC element. The cut-off frequency fg of a filter stage with an RC element depends on the RC time constant of the filter stage. The cut-off frequency fg of the associated filter stage therefore also varies as a result of fluctuations in the resistances and capacitances.
In order to avoid any fluctuation in the cut-off frequency fg of the filter stage, the filters are therefore tuned or trimmed after production.
FIGS. 1a to 1c show various possible ways for indirect tuning or for trimming of filters.
In FIG. 1a, an on-chip resistor R0, which, for example, is in the form of an MOS transistor, is compared by means of a comparison circuit with an external high-precision resistance Rext. The value of the on-chip resistor R0 is automatically readjusted by means of a monitoring voltage Vc until the on-chip resistor R0 has precisely the same value as the high-precision external resistance Rext.
In the arrangement as illustrated in FIG. 1b, a reference filter is used which has the same circuitry as the main filter that is to be adjusted. A sinusoidal signal is fed into the reference filter, and a phase comparator continuously compares the phase at the output of the main filter with that of the sinusoidal signal. The monitoring voltage Vc is readjusted until the desired phase value is reached.
A further possible way to carry out trimming according to the prior art comprises, as is illustrated in FIG. 1c, the use of an oscillator as a reference circuit. This oscillator is formed from the same circuit elements as the main filter. A comparison circuit compares the phase angle of the output signal emitted from the oscillator with that of the injected sinusoidal signal.
A monitoring voltage is readjusted until the desired phase value is reached.
The tuning methods that are illustrated in FIGS. 1a to 1c are based on tight matching of components on the chip, which are at a very short distance from one another.
FIG. 2 shows a so-called biquad filter according to the prior art. Biquad filters are second-order analog filters which are completely differential and contain two filter stages. Each filter stage in the biquad filter illustrated in FIG. 2 has an RC element, in each case comprising a resistor R and a capacitor C. The resistances and capacitances are subject to manufacturing tolerances after the production process. These manufacturing tolerances may, for example, be plus/minus 20% in the case of capacitors, and plus/minus 15% in the case of resistors. These production discrepancies result in the time constants (τ=R×C) of the various filter stages varying widely, and in the cut-off frequencies fg being shifted. This in turn leads to a change in the respective cut-off frequency fg of the filter. During the integration of the second-order biquad filter as illustrated in FIG. 2 and according to the prior art, this filter must therefore be trimmed or tuned.
FIG. 3 shows a block diagram of a circuit arrangement for trimming a filter according to the prior art. Conventional tuning and trimming circuits are integrated on the chip, as an autonomous circuit block. This separate trimming circuit is located on the same chip as the analog filter to be tuned and as the other functional circuit components. The trimming circuit according to the prior art essentially comprises an analog circuit for measurement of the RC time constant (τ) of a filter stage within the filter, and a digital circuit for evaluation of the measured time constant τ. The analog RC measurement circuit within the separate trimming circuit is a circuit arrangement which is identical to the filter stage, which is to be measured and to be trimmed, within the filter. The digital circuit part of the trimming circuit evaluates the measured time constant τ of the filter stage and switches a capacitor array, which is provided in the filter, in order to compensate for any discrepancy between the RC time constant and a nominal value.
The circuit arrangement as illustrated in FIG. 3 and according to the prior art for trimming of an analog filter has considerable disadvantages. The trimming circuit is arranged on the chip separately from the filter to be trimmed. Since the trimming circuit contains a model of the circuitry of the filter stage to be trimmed, it requires virtually just as much space during integration on the chip as the filter to be trimmed itself. The additional space required leads to considerably higher production costs for the overall chip.
Since the trimming circuit is integrated on the chip separately from the filter to be trimmed, this results in a physical separation between the RC elements within the filter stage to be trimmed and the RC elements within the trimming circuit, which identically model the filter stage. The physical separation means that, from a circuitry point of view, good matching between the impedances of the capacitors within the filter to be trimmed and between the impedances of the capacitors within the RC measurement circuit which is contained in the trimming circuit can be achieved only with difficulty. Manufacturing differences and temperature gradients may lead to the RC elements within the trimming circuit and within the filter stage to be trimmed having a different behavior. This in turn results in the filter stage being tuned or trimmed incorrectly.